1. Field of the Invention
The present invention relates to a suck back valve which, for example, prevents liquid drip from occurring in a fluid supply port, by sucking back a fluid which flows through a fluid passage in accordance with a displacement action of a diaphragm, yet which is capable of stabilizing the suck back amount of the fluid.
2. Description of the Related Art
The suck back valve has been hitherto used, for example, in a production process for forming semiconductor wafers. The suck back valve has a function to prevent so-called liquid drip, i.e. a phenomenon in which a minute amount of coating liquid drips toward the semiconductor wafer from a supply port when the supply of coating liquid to the semiconductor wafer is stopped.
A suck back valve according to a conventional technique is shown by FIG. 5, further details of which may be seen, for example, in Japanese Utility Model Publication No. 8-10399.
The suck back valve 10 includes a main valve body 18 formed with a fluid passage 16 for making communication between a fluid-introducing port 12 and a fluid-discharging port 14, and a bonnet 20 coupled to an upper portion of the main valve body 18. A diaphragm 26, which comprises a thick-walled section 22 and a thin-walled section 24, is provided at a central portion of the fluid passage 16, wherein the overall surface of the thick-walled section 22 and thin-walled section 24 of the diaphragm is formed so as to face the fluid passage 16.
A piston 30 is fitted to the diaphragm 26, wherein a v-packing 32, which is slidable on an inner wall surface of the main valve body 18 and which functions as a seal, is installed on the piston 30. Further, a spring 34, which constantly presses the piston upwardly, is provided in the main valve body 18. The bonnet 20 is formed with a pressurized air supply port 28, wherein the pressurized air supply port 28 is connected to a pressurized air supply source (not shown) through an unillustrated flow amount control valve or the like. Reference numeral 36 indicates a screw member which abuts against the piston 30 to adjust the displacement amount of the piston 30, wherein the flow amount of a coating liquid sucked by the diaphragm 26 can be adjusted.
Operation of the suck back valve 10 shall now be explained in outline. In an ordinary state in which a coating liquid is supplied from the fluid-introducing port 12 to the fluid-discharging port 14, the flow amount control valve or the like is controlled, wherein pressurized air is supplied to the pressurized air supply port 28 from the pressurized air supply source. As a result, the piston 30 is displaced downwardly in accordance with the pressure of the pressurized air, and the diaphragm 26 which is coupled to the piston 30 protrudes into the fluid passage 16, as shown in FIG. 5 by the two-dot chain lines.
When flow of coating liquid through fluid passage 16 is halted, the piston 30 and diaphragm 26 are raised in unison under the action of the elastic force exerted by the spring 34, by controlling the flow amount control valve, or the like, and stopping the supply of pressurized air supplied to the pressurized air supply port 28 from the pressurized air supply source. Along with controlling such displacement by abutment of an end of an adjustment screw 36, a predetermined amount of coating liquid remaining in the fluid passage 16 is sucked under action of a negative pressure produced by the diaphragm 26. Thus, dripping of coating liquid, which would otherwise be caused at a fluid supply port connected to the fluid-discharging port 14 is prevented.
Notwithstanding, with the conventional suck back valve described above, a flow control valve, or the like, is needed for controlling the flow amount of pressurized fluid supplied to the pressurized air supply port 28, and piping operations for connecting the suck back valve 10 and the flow control valve introduce complications. Further, there is a disadvantage that a dedicated space for the system which employs the suck back valve is enlarged. Further, pressurized air supplied to the pressurized air supply port 28 is controlled by the flow control valve, and in order to cause movement of the piston 30 and displacement of the diaphragm 26 by pressure of the pressurized air, time is required from initiating control of the flow control valve until displacement of the diaphragm actually occurs, lengthening the response speed of the diaphragm 26. As a result, there is the concern that a delay occurs in the operation from halting supply of the coating liquid until the coating liquid is sucked back, and that more than a desired amount of coating liquid drips onto the semiconductor wafer.
A structure has been contemplated in which, in order to hasten the response speed of the diaphragm 28, the diaphragm 28 is forcibly displaced using an unillustrated electric actuator. However, in the case that applied power to the electric actuator is small, when the actuator is activated and the diaphragm 26 is displaced toward the interior of the fluid passage 16, the diaphragm is also pressed by the pressure of the coating liquid introduced to the inside of the fluid passage 16, and the diaphragm remains depressed in a direction opposite to that of the applied force from the electric actuator. Moreover, when supply of the coating liquid is halted, the diaphragm 26 is displaced and the coating liquid inside the fluid passage 16 is sucked, however, because the diaphragm is already in a condition of displacement, the coating liquid cannot be sufficiently sucked and there is the concern that coating liquid will drip onto the semiconductor wafer. Further, in order to prevent this type of problem, a large scale electric actuator which generates a large force is required, and as a result, there is the disadvantage that the suck back valve itself becomes large in scale.
In addition, in order to displace the diaphragm 26 by the compressed air supplied to the pressurized air supply port 28, when the pressure of the pressurized air supplied from the pressurized air supply source varies, the piston displacement amount is changed, and the coating liquid amount which is sucked by the diaphragm also changes. As a result, when supply of the coating liquid is halted, coating liquid remaining inside the fluid passage 16 is not sufficiently sucked, and unwanted coating liquid drips onto the semiconductor wafer. On the other hand, if the amount of sucked coating liquid becomes greater than desired, at the start of supply of the coating liquid, there is a problem that time is required until the coating liquid begins to drip onto the wafer.